Some aerospace systems, such as the International Space Station (ISS) and the Space Transportation System (STS), or Space Shuttle, produce large volumes of telemetry which must be transmitted to terrestrial stations for use by mission controllers and vehicle health managers, among others. The transmission occurs over multi-nodal, multi-tiered networks which include nodes within and exterior to the space vehicles. Because bandwidth for transmitting the telemetry data is limited, various approaches to bandwidth compression have been attempted, one of which is the transmit-by-exception approach.
In a transmit-by-exception telemetry data transfer, the only data transmitted are those telemetry values which have undergone a significant change since they were last transmitted. This approach can provide substantial bandwidth compression. A resulting difficulty, however, is that the user on the ground cannot tell the difference between data that is not changing because it has not required transmission and data that is not changing because some portion of the network is malfunctioning. Network anomalies may be intermittent, making some telemetry faulty and leaving some telemetry valid. Unchanging data is ambiguous at the point of reception as to the cause for the lack of change.
The problem is exacerbated when the telemetry data is to be used in Integrated Vehicle Health Management Systems (IVHMSs). IVHMSs access telemetry data for use by diagnostic and prognostic software to support vehicle health maintenance. Experience has shown that ambiguous telemetry data may cause known IVHM algorithms to produce erroneous results.
One approach is to observe a constantly-changing telemetry data element, or counter, transmitted from the same source node as the transmit-by-exception data to be disambiguated, or target data. The target data may be evaluated as valid if the frequently-changing telemetry data element is seen to change over the time period when the target data was sent. The approach has several weaknesses. First, in a multi-path, multi-tiered communications network signal noise can cause false data values in the counter data, leading an algorithm to conclude that the counter is operating when, in fact, it is not operating. Thus, target data that is invalid may be erroneously seen as valid.
Second, counters that change at different rates on different nodes are utilized. The temporal resolution of a pairing disambiguation scheme for an individual node is the period of that node's counter minus the pulse width of the data bit. The temporal resolution of the disambiguation scheme for the network as a whole is the resolution of the slowest counter in the network. The temporal resolution for the network as a whole matters because IVHM systems need a series of complete “snapshots” of the vehicle system that give, as closely as possible, the state of the vehicle at particular times. If some of the data has gone bad without notice, the snapshots will be flawed and the IVHM system will reach an erroneous conclusion.
Designers and operators of large networks such as those used with ISS and STS often use their own commercial-off-the-shelf (COTS) equipment. Counters of different frequencies are inevitable, and the counter frequency for a particular node may be selected based upon vendor-unique or situational criteria rather than a desire for uniformity. Ideally, each node might be equipped with a high-speed clock, but this would quickly recreate the bandwidth-saturation problem that transmit-by-exception telemetry was designed to solve. For some existing networks, retrofitting each network node with a counter would be impractical. If one counter in a network operates at a frequency of 0.1 Hz, it could be nearly 10 seconds before a problem becomes apparent. Disambiguation schemes with low temporal resolutions are problematic in IVHM systems, so the pairing approach is rejected.
Networks are reconfigurable by nature, making the process of tracking network health an expensive and energetic undertaking. For example, whenever a new module is added to the ISS, at least one sub-net must be added to the network and new telemetry points must be added. The same may be true for additions, such as a robotic arm, incorporated into an STS.
Accordingly, it is desirable to disambiguate transmit-by-exception telemetry to improve the performance of IVHM and other systems. It is therefore desired to avoid poor temporal resolution and noise-induced false validations. In addition, it is desirable to provide a user-friendly process of providing telemetry disambiguation that is adaptable to changes in the structure of the network and the set of telemetry variables sent over the network. Avoiding physical upgrades to existing network equipment while creating the desirable features just mentioned is also desirable. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.